Combined cycle power plants are well known in the art. A combined cycle power plant includes both a gas turbine-based topping cycle and a steam turbine or a steam rankine bottoming cycle that is driven by heat in the exhaust of the gas turbine engine. U.S. Pat. No. 6,145,295, assigned in common to the assignee of the present invention, describes one exemplary combined cycle power plant.
FIG. 1 is a simplified schematic of an exemplary combined cycle power plant 10 that may benefit from aspects of the present invention. Heat contained in an expanded working medium (e.g., flue gas) from at least one gas turbine (GT) 11 is utilized to generate steam for one or more steam turbines (e.g., steam turbines 12 and 14). The heat transfer is effected in a heat-recovery steam generator (HRSG) 16, which is connected downstream of the gas turbine on the flue-gas side and in which heating areas are disposed in the form of tubes or banks of tubes. The latter in turn are connected by way of a water/steam circuit to the steam turbines. The water/steam circuit may include a plurality of pressure stages, for example three pressure stages.
The steam generated in the heat-recovery steam generator 16 is fed to the steam turbines 12 and 14, where it expands to perform work. The steam turbines may include a number of pressure stages, which are adapted in their number and layout to the structure of the heat-recovery steam generator. The steam expanded in the steam turbines may be fed to an air-cooled condenser (ACC) 18 to be condensed therein. The size and cost of the ACC 18 is not insignificant considering that the size of such a condensing device may be as large as a football field. The condensate resulting during the condensation of the steam is returned as feedwater to the heat-recovery steam generator 16 so that a closed water/steam circuit is obtained.
For air-cooled combined cycle power plants, such as may be deployed in relatively dry geographical regions where water is a scarce resource, one of the most severe sizing constraints for the ACC arises due to the requirement of being able to perform a plant startup or restart during a period of peak thermal load, such as on a hot day. Typically, during plant startup, gas turbine load is ramped upward with essentially all steam production from the heat recovery steam generator bypassing the steam turbines 12 and 14 to the ACC 18. Because there is no energy removed from the steam by the steam turbine during bypass operation, the heat duty imposed on the ACC 18 is relatively high, resulting in relatively higher steam turbine exhaust pressure. As gas turbine load and the resulting heat duty to the condenser increase, so does the steam turbine exhaust pressure. In order to bring the steam turbine on line, however, steam turbine exhaust pressure should be maintained at a relatively low value to avoid exceeding steam turbine design limits during the startup process.
Plant startup on a hot day aggravates the situation further because high ambient temperatures lead to even higher steam turbine exhaust pressures, as the ACC performance depends significantly on the temperature difference (Tsaturated−Tambient). Hence, it is even more difficult on hot days to maintain the steam turbine exhaust pressure below the maximum allowable value for rolling the steam turbine.
Other factors in today's power generation market play a roll in defining requirements for plant startup. For instance, plant operators generally place greater emphasis on reducing plant startup time and overall plant emissions. This results in a need to quickly ramp the gas turbines to relatively high load levels prior to rolling the steam turbine, which makes maintaining low steam turbine back pressure prior to rolling the steam turbine very difficult.
One conceptually possible approach for meeting these peak heat transfer requirements for plant startup or restart on a hot day is to increase the size of the ACC 18. In a practical implementation, such an approach would significantly increase the size and cost of the ACC without a corresponding improvement in base load plant performance and, as a result, the overall combined cycle power plant would be less cost effective. Accordingly, it is desirable to provide an improved air-cooled plant and condensing apparatus that can accommodate the various operating criteria for plant startup or restart during a period of peak thermal load while achieving a reduction in the cost of the overall air-cooled power plant.